1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a facsimile machine and a printer, and more particularly to an improvement of a charging device that charges the surface of a latent image carrier uniformly.
2. Description of the Related Art
Generally, an electrophotographic image forming apparatus creates an image by the following processes. Specifically, first, a latent image carrier, such as a photoreceptor, that is charged uniformly is subjected to exposure scanning and the like to form an electrostatic latent image thereon, and this electrostatic latent image is developed by a developing device. Next, a toner image obtained by development is transferred from the latent image carrier to a transfer sheet or other recording body directly or via an intermediate transfer body.
As the image forming apparatus having such a configuration, the one described in, for example, Japanese Unexamined Patent Publication No. 2003-316202 is known. In this image forming apparatus, a toner image that is formed on a rotating surface of a drum-like photoreceptor, i.e., a latent image carrier, is primarily transferred to an intermediate transfer belt at a primary transfer nip formed by abutment of the photoreceptor with the intermediate transfer belt. Then, the surface of the photoreceptor is charged uniformly by a charging device after the toner image passes through the primary transfer nip. A transfer residual toner adheres to the surface of the photoreceptor after the toner image passes through the primary transfer nip, and a so-called cleaner-less system is employed in which the transfer residual toner within the developing device is recovered after the surface of the photoreceptor is charged uniformly, without removing the transfer residual toner.
Various methods for realizing the cleaner-less system are known. The conventional image forming apparatus described in the abovementioned publication adopts the following method. Specifically, first, the transfer residual toner that remains on the surface of the photoreceptor after passing through the primary transfer nip is trapped by a first brush that abuts against the photoreceptor while applying a bias having a polarity opposite to the normal charging polarity of the toner. Then, the transfer residual toner is gradually charged to the polarity opposite to the normal charging polarity within the first brush, and thereafter the transfer residual toner is slowly shifted to the photoreceptor having a polarity same as the normal charging polarity of the toner. In this manner, the transfer residual toner is temporarily trapped within the first brush and thereafter shifted slowly to the receptor, whereby it is possible to avoid defective charging that occurs when performing uniform charging on the photoreceptor while keeping a large quantity of transfer residual toner adhered thereto. The transfer residual toner that is shifted to the photoreceptor is charged again to the normal charging polarity by a second brush that abuts against the photoreceptor while applying a bias having a polarity same as the normal charging polarity of the toner. Then, the transfer residual toner is caused to pass through an abutting portion between the photoreceptor and a charging roller charging the photoreceptor uniformly, thereafter shifted to a developing sleeve at a developing region where the photoreceptor faces the developing sleeve of the developing device, and then recovered in the developing device. According to the abovementioned publication, the cleaner-less system can be realized by the series of steps described above.
However, according to an experiment performed by the three inventors of the present invention, the transfer residual toner, which has been trapped within the first brush applying the bias having a polarity opposite to the normal charging polarity of the toner, could not be transferred to the photoreceptor properly, whereby a large quantity of transfer residual toner was accumulated in the first brush. Then, the large quantity of accumulated transfer residual toner was shifted from the first brush to the photoreceptor at irregular times, causing defective charging of the photoreceptor.
The reason that the large quantity of transfer residual toner was accumulated in the first brush is as follows. Specifically, the polarity of the transfer residual toner trapped within the first brush is the normal charging polarity. In order to charge this transfer residual toner to the opposite polarity within the first brush, it is necessary to inject charges or discharge electricity from the first brush into toner particles. Moreover, by performing charge injection or electric discharge, it is necessary to apply a large amount of charges to the toner particles so that the polarity of the transfer residual toner is reversed.
However, since a plurality of bristles configuring the first brush and the toner particles are not closely attached to one another so well within the brush, it is difficult to perform good charge injection so as to move a large amount of charges in a short amount of time. Furthermore, there are quite a few toner particles that cannot be subjected to charge injection itself due to poor adhesion with the bristles. Therefore, it is difficult to reverse the polarity of the transfer residual toner trapped within the first brush, by performing charge injection. In addition, if the first brush is applied with a bias of a value high enough that electricity is discharged from the bristles, it means that this electric discharge is caused mainly between the brush and the photoreceptor, thus the charges obtained from this electric discharge pass through the toner particles and move to the photoreceptor. Therefore, it is difficult to reverse the polarity of the transfer residual toner trapped within the first brush, by means of the electric discharge from the bristles.
For these reasons, the transfer residual toner could not be shifted properly from the first brush to the photoreceptor.
The above has described the problems of the image forming apparatus using a cleaner-less system, but even in the system for cleaning the transfer residual toner by using cleaning means, a large quantity of transfer residual toner that can not be removed completely by the cleaning means might be accumulated in the first brush.